1. Technical Field
The present invention relates to a sheet feeding apparatus that separates and feeds one sheet at a time from a stack of sheets placed on a table for sheets, and to image forming apparatuses.
2. Background Invention
In recent years, air sheet feeding apparatuses that can feed a large quantity of sheets are being used in image forming apparatuses such as copying machines, printers, etc. An air sheet feeding apparatus is one in which a sheet is made to float up by blowing air on to the side surface of a stack of sheets, a sheet is made to get sucked on to the suction surface of a belt, etc., in which are formed air suction holes that suck in air, and the sheet is conveyed by driving the belt in a rotating manner.
Conventionally, this kind of sheet feeding apparatus was one that fed large quantities of sheets of the same size or of the same type. However, in recent years, due to the progress in digital information technology, a printing method called print on demand has come into widespread use in which the data prepared by a computer, etc., is printed out directly without going through an intermediate step of preparing offset printing films or plates, and sheet feeding apparatuses have become necessary that can feed sheets of various types and sizes that are being used in the printing industry.
In Examined Japanese Patent Application Publication No. 3855512 (Patent Document 1), an air sheet feeding apparatus has been disclosed wherein, nozzles (sheet raising means) that jet out air on the front surface of a stack of sheets for causing a sheet to float up, and nozzles (sheet separating means) for blowing air for separating other sheets from a single sheet that is made to be sucked and attached to the conveying belt are provided on the front side of the stack of sheets in the direction of feeding the sheet.
According to Patent Document 1, sheet feeding is made possible without feeding several sheets simultaneously by selectively switching the air from the sheet raising means and the sheet separating means according to the timing of sheet feeding by the conveying belt.
In Examined Japanese Patent Application Publication No. 3891405 (Patent Document 2), an air sheet feeding apparatus has been disclosed wherein, a sheet at the topmost surface of a stack of sheets is fed while being sucked by a suction and conveyance means because the sheet is floated up by a first air blowing means that blows air on the front surface of a stack of sheets from the front side of the direction of sheet feeding, and a second air blowing means that blows air on the side surface of the stack of sheets.
According to Patent Document 2, a positioning member is provided that restricts the height on the side surface of a sheet or in its neighborhood, a constant sheet separation is obtained at all times by selecting the air blow from a second air blowing means in accordance with the size of the sheet.
However, in an air sheet feeding apparatus described in Patent Document 1 or Patent Document 2, in order to separate definitely the topmost sheet that is sucked by the sucking and conveying section from the other sheets, a certain amount of space is required in the up-down direction between the topmost surface of the stack of sheets and the suction and conveyance section.
In this kind of configuration, in the floating process of transiting from the stacked state to the state of getting sucked by the sucking and conveying section, since the raised sheet can take various types of postures (such as flexure or distortion) depending on the condition of the air getting into the stack of sheets, conditions can arise in which the leading edge of the sheet is sucked by the suction and conveyance section at an angle to the conveying direction. If a sheet is sucked and conveyed in this condition, the suction area moves towards the trailing edge so as to maintain the position of the leading edge of the sheet that is inclined. In other words, the sheet gets skewed.
As described above, an air sheet feeding apparatus has the problem that the fluctuations in the sheet skewing are large, and improvement of sheet skewing is desired. In particular, the demand is very strict about the positional accuracy of the image formed on a sheet and the sheet (image registration) in the field of print on demand, and improvement of sheet skewing is strongly desired in the case of air sheet feeding apparatuses used in this field.
In an image forming apparatus, although a skewing correction mechanism (paper registration mechanism) is provided at a position just before image transfer, particularly in the case of sheets of large sizes, since the part of the sheet on the upstream side is nipped by a plurality of conveying rollers, even if an attempt is made to correct a large skewing at this point of time, due to the pulling of the sheet between the registration mechanism and the conveying rollers, a large stress is applied on the sheet, problems occur such as buckling or wrinkling, etc.
An object of the present invention is to solve the problem that the fluctuations in sheet skewing are large, and to provide an air sheet feeding apparatus having excellent sheet feeding performance that is demanded in the print on demand field.